To investigate the feasibility of utilising preoperative contrast-enhanced CT data to estimate renal split function, compared to standard-of-care nuclear medicine studies - Tc-99m dimercaptosuccinic acid scan (DMSA) or Tc-99m mercaptoacetyltriglycine scan (MAG3) - in patients planned for nephrectomy.
Following institutional review board waiver of consent, a retrospective analysis was conducted on patients with either DMSA- or MAG3-derived estimates of split renal function (NM-SRF) and a contemporaneous contrast-enhanced CT scan (within 3 months) performed in the work-up for nephrectomy. Patients with hydronephrosis were excluded. Using reconstruction software with density thresholding to exclude urine and calcification (Syngo.via, Siemens Healthineers), renal volumes and mean attenuation values were calculated. Renal tumours were excluded from the volume. The product of volume and mean attenuation from each kidney were compared to calculate a CT-derived estimate of split renal function (CT-SRF). The estimated percentage renal function from the right kidney from NM-SRF and CT-SRF were compared using Pearson correlation coefficient. In patients undergoing nephrectomy, the estimated remaining renal function (glomerular filtration rate - GFR) following nephrectomy using NM-SRF and CT-SRF was compared to actual GFR values obtained >3 months post-operative. The difference between CT-SRF and NM-SRF and the true post-operative GFR were compared using Student’s T-test.
Coronal portal venous phase CT scan showing a left lower pole renal tumour and relative thinning of the right renal cortex. (b) volumetric rendering of the segmented kidneys. Note that the exophytic left renal tumour was not included in the volume-of-interest. Left renal volume was 171.4cm3 with mean enhancement 87 HU (right 48.9cm3, 82 HU), estimating 21.2% function from the right kidney (c) DMSA study, estimating 21% function from the right kidney.
n=49 patients (mean age 65.8 ± 9.3 years; 73.4% male) were included in the initial analysis. The Pearson correlation coefficient between CT-SRF and NM-SRF was 0.835, indicating a very strong correlation (p=3 months following the date of nephrectomy) was available. Following 3 exclusions (n=2 due to nature of surgery, n=1 due to critical illness at the time of renal function testing), n=28 patients were included in the second analysis. CT-SRF and NM-SRF underestimated post-operative GFR by 12.1 (±11.9) and 10.5 (±12.2) ml/min/1.73m2, respectively (Figure 2). There was no significant difference in performance between these tests (p=0.604) Following exclusion of the first five cases (to allow for familiarisation with the software), the mean time to calculate CT-SRF was 157±68 seconds.
Estimated percentage function from the right kidney, comparing CT-derived estimates against standard-of-care nuclear medicine tests. Pearson correlation coefficient 0.835 indicating very strong correlation.
Absolute difference between estimated and true post-nephrectomy renal function, calculated using CT- and nuclear medicine-derived estimates of split renal function and post-operative GFR values. A positive value indicates that true post-operative GFR was better than predicted. Almost all patients had better true post-operative renal function than was predicted by both CT- and NM-derived estimates.
CT-SRF is shown to be a time-efficient and feasible way of estimating split renal function with comparable performance to standard-of-care NM-SRF in this small retrospective cohort. Larger prospective studies are required to assess whether it is possible to use already-acquired CT data to avoid the additional radiation exposure, hospital attendance, and cost associated with DMSA and MAG3 testing.
